That's odd. Is /dev or / mounted read-only? What about the modes
on /dev/ad8s2d ? If you do a fresh boot (i.e. never having tried to
mount anything using that device), and then issue the dd, does it
work differently?

Does disklabel recognize the disk?

Also try regenerating the /dev entry for ad8s2.

cd /dev
./MAKEDEV ad8
./MAKEDEV ad8s2
./MAKEDEV ad8s2a

Hmm,

I tried most of this, all negative.

However, I _do_ have some partitions of the ad8s1 slice mounted (/,
/usr, etc.). Could it be because of this? I would think this shouldn't
affect partitions on ad8's other slices, right?